Simple. Monitor the voltage of the front O2 sensors with the engine at operating temp. Hold the RPM at around 2,500 until the sensor voltage fluctuates normally. Then, quickly stab the throttle to the floor and lift immediately. On decel, the sensor voltage should drop to 0 volts (or darn close). If the sensors can't reach 0 volts it can cause the air injection system to fail the "self-test". It's actually pretty common on older vehicles. The nice thing about this test is that it only takes a few minutes to do, and doesn't require any parts.